1. Field of the Invention
The invention relates to a piezoelectric resonant filter including a thin-film piezoelectric resonator, a duplexer including the resonant filter, and manufacturing methods thereof.
2. Description of the Related Art
Mobile communications devices such as mobile phones, which have dramatically become widespread in recent years, have been miniaturized and have attained higher usable frequencies year by year. In response to this, it is desired that electronic components for use with such mobile communications devices also be miniaturized and made usable at higher frequencies.
Some mobile communications devices include a duplexer for switching between a transmission signal path and a reception signal path in order to share a single antenna for both transmitting and receiving signals. The duplexer has a filter for allowing a transmission signal to pass therethrough and interrupting a reception signal, and a filter for allowing the reception signal to pass therethrough and interrupting the transmission signal.
In recent years, some of the aforementioned duplexers incorporate a surface acoustic wave filter. The surface acoustic wave filters have features that they are usable at frequencies up to about several gigahertz and can be made more compact than ceramic filters. However, usable frequencies of mobile communications devices can become even higher in the future, while under the present situation there remain many technical problems for the surface acoustic wave filters to become usable at such high frequencies.
In this regard, attention has been focused lately on a thin-film piezoelectric resonator, which is also called a thin-film bulk acoustic resonator (hereinafter also referred to as FBAR). The thin-film piezoelectric resonator utilizes resonance of a piezoelectric thin film in a direction of its thickness. Changing the thickness of the piezoelectric thin film can change the resonant frequency of the thin-film piezoelectric resonator. The thin-film piezoelectric resonator is expected to be capable of responding to frequencies up to several gigahertz.
Such a thin-film piezoelectric resonator is described, for example, in the following documents:
Published Unexamined Japanese Patent Application (KOKAI) No. 2000-278078;
U.S. Pat. No. 5,872,493;
U.S. Pat. No. 4,642,508;
U.S. patent application Publication No. 2002/0070262 A1; and
a paper entitled xe2x80x9cThin Film Resonators and Filtersxe2x80x9d, Kiyoshi Nakamura et al., presented at International Symposium on Acoustic Wave Devices for Future Mobile Communication Systems, Mar. 5-7, 2001, pp. 93-99.
A thin-film piezoelectric resonator comprises a piezoelectric thin film, two electrodes disposed on both surfaces of the piezoelectric thin film, and a base body for supporting the piezoelectric thin film and the electrodes. The base body may have a cavity which forms an opening on a side opposite to the side on which the piezoelectric thin film and the two electrodes are disposed (see Published Unexamined Japanese Patent Application (KOKAI) No. 2000-278078 and U.S. Pat. No. 5,872,493). Otherwise, a gap may be formed between one of the electrodes and the base body (see U.S. Pat. No. 4,642,508). Otherwise, the piezoelectric thin film and the two electrodes may be disposed on an acoustic multi-layered film on the base body, without cavity or gap (see Kiyoshi Nakamura et al.).
Filters employing a resonator include a ladder filter. A ladder filter includes a series resonator and a parallel resonator, as a basic configuration. As necessary, a plurality of the basic configurations are connected in a cascaded manner to make up a ladder filter.
Now, for example, suppose that a filter including a plurality of resonators such as the aforementioned ladder filter is packaged. In this case, a chip including filter components is formed and then the chip is mounted onto a mounting substrate to thereby fabricate a packaged filter.
Conventionally, to package a filter including a plurality of resonators, a wire bonding method has been widely employed for establishing electrical connection between the chip and the mounting substrate (see U.S. Pat. No. 5,872,493).
Now, a method for establishing electrical connection between the chip and the mounting substrate by wire bonding will be briefly described. In this method, the chip including the base body and elements mounted thereon is placed on the mounting substrate such that the surface of the base body having the elements mounted thereon faces upward, and then bonded onto the mounting substrate using an adhesive or the like. The chip is provided with connection electrodes, and the mounting substrate is provided with connection pads. The connection electrodes and the connection pads are connected to each other with thin metal wires. For example, the thin metal wires are 20 to 30 xcexcm in diameter. The thin metal wires are made of a material such as gold or aluminum. Connection between the thin metal wires and the connection electrodes, or connection between the thin metal wires and the connection pads is established by thermo-compression bonding, ultrasonic bonding, or a combination thereof.
When the electrical connection between the chip and the mounting substrate is established by wire bonding, the thin metal wires cause an extra parasitic inductance between the chip and the mounting substrate. The extra parasitic inductance caused by the aforementioned thin metal wires may result in a shift of the electrical properties of the filter from desired ones, even when the chip has been fabricated so as to obtain the desired electrical properties of the filter. The electrical properties of the filter include frequencies for determining the passband of the filter, center frequency of the passband, insertion loss, and the amount of attenuation in an attenuation band.
For conventional filters used in a frequency band of 1 GHz or less, for example, a shift of the electrical properties of the filters caused by the aforementioned extra parasitic inductance is slight and has been neglected.
However, for filters used in a high frequency band of several GHz to the order of 10 GHz such as piezoelectric resonant filters employing a thin-film piezoelectric resonator, a shift of the electrical properties of the filters caused by the aforementioned extra parasitic inductance is non-negligibly great.
To avoid such a problem as mentioned above, as a method for establishing electrical connection between the chip and the mounting substrate, it is proposed that a solder bumps provided on the chip are directly connected to the connection pads of the mounting substrate by flip chip bonding (see U.S. patent application Publication No. 2002/0070262 A1).
Now, briefly described is an example of a method for establishing electrical connection between the chip and the mounting substrate by the aforementioned flip chip bonding. In this method, first, fine solder bumps having a diameter of several tens to 100 xcexcm or so are formed on the connection pads provided on the chip by using a high-melting solder. On the other hand, the connection pads provided on the mounting substrate are pre-coated with solder by using a procedure such as plating, thick-film printing of solder paste, and vapor deposition. The solder bumps are then soaked in flux. The chip is then positioned and mounted on the mounting substrate such that the solder bumps of the chip face the connection pads of the mounting substrate. Then, with this state remained unchanged, the solder with which the connection pads of the mounting substrate has been pre-coated is melted by using a reflow furnace or the like. After that, the solder is solidified so that the solder bumps of the chip and the connection pads of the mounting substrate are electrically and mechanically bonded to each other. Then, the flux is cleaned off as required. Then, to improve reliability of the connection between the solder bumps of the chip and the connection pads of the mounting substrate, an underfill resin may be filled in between the chip and the mounting substrate followed by hardening of the underfill resin.
The flip chip bonding allows high-density packaging and is therefore widely employed for packaging of electronic components used in computers, and so on. The flip chip bonding allows the connection pads of the chip to be electrically connected to the connection pads of the mounting substrate via fine bumps. Therefore, by establishing electrical connection between the chip and the mounting substrate in a piezoelectric resonant filter through the use of the flip chip bonding, an extra parasitic inductance occurring between connection pads of the chip and the mounting substrate can be significantly reduced.
In prior art, as a method for establishing electrical connection between the chip and the mounting substrate in a surface acoustic wave filter, the following method has been proposed as described in Hiromi Yatsuda et al., xe2x80x9cMiniaturized SAW Filters Using a Flip-Chip Techniquexe2x80x9d, IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 43, NO. 1, pp. 125-130, JANUARY 1996. In this method, first, metal bumps made of Au, for example, are formed on the connection pads of the chip using a conventional wire bonding machine. At this time, as required, the chip may be heated by using a heating stage to accelerate bonding of the metal bumps to the connection pads. The chip is then positioned and mounted on the mounting substrate such that the metal bumps of the chip face the connection pads of the mounting substrate. Then, the chip is subjected to an ultrasonic wave under pressure to bond the metal bumps to the connection pads of the mounting substrate. At this time, the chip may be heated to thereby accelerate the bonding. In this method, the metal bumps are bonded to the connection pads of the mounting substrate by interdiffusion between the metal atoms that make those bumps and connection pads in a solid phase.
As a method for establishing electrical connection between the chip and the mounting substrate, the technique of directly connecting the solder bumps of the chip to the connection pads of the mounting substrate by flip chip bonding has been widely employed because of its high reliability. However, from the viewpoint of friendliness to the environment, it is not preferable to use solder that contains lead.
Moreover, when this method is employed, in the step of melting the solder by heating, the flux can be scattered over the device surfaces of the chip to thereby contaminate the device surfaces. For this reason, there is typically provided a cleaning step for removing flux residues after the solder bumps of the chip have been bonded to the connection pads of the mounting substrate. Nevertheless, after the cleaning step, flux residues and impurities such as the flux melted in the cleaning solution may remain sticking to the device surface.
In a thin-film piezoelectric resonator, bulk waves occur in the piezoelectric thin film by application of a high frequency voltage to the two electrodes disposed on both surfaces of the piezoelectric thin film. The resonant state and the anti-resonant state of the thin-film piezoelectric resonator depend on the thickness of the piezoelectric thin film. Therefore, in a thin-film piezoelectric resonator, even a slight contamination of the surface of the device or the thin-film piezoelectric resonator would result in variations in the resonant frequency and the anti-resonant frequency of the thin-film piezoelectric resonator due to a mass loading effect. Any shifts of the resonant frequency and anti-resonant frequency of the resonator in the filter from the originally designed values would cause the center frequency of the passband of the filter to be shifted from its originally designed value. Furthermore, any shifts of resonant frequency and anti-resonant frequency of the resonator in the filter from the originally designed values would probably increase the insertion loss of the filter.
To prevent such contamination of the surface of the thin-film piezoelectric resonator as mentioned above, it is conceivable to provide an acoustic mirror on the resonator to protect it, as described in U.S. Pat. No. 5,872,493. In this case, however, the acoustic mirror made up of a plurality of layers need to be formed with high precision in thickness, which makes the manufacture of the thin-film piezoelectric resonator difficult.
To perform flip chip bonding without solder, a method is known in which gold bumps provided on the chip are electrically connected to the connection pads of the mounting substrate by using a conductive paste or an anisotropic conductive sheet. However, the method employing the conductive paste leads to a lower reliability of the electrical connection between the bumps and the connection pads, or requires a long time for drying the conductive paste. On the other hand, in the method employing the anisotropic conductive sheet, the anisotropic conductive sheet contacts the surface of the chip on which devices are mounted. Therefore, this method is not preferable for use for mounting the chip in piezoelectric resonant filters.
As described above, in the prior art, piezoelectric resonant filters incorporating a thin-film piezoelectric resonator have suffered from problems such as occurrence of shifts or degradation of the electrical properties of the filter related to mounting of the chip onto the mounting substrate, and a poor reliability of the electrical connection between the chip and the mounting substrate.
It is therefore an object of the invention to provide a piezoelectric resonant filter comprising a chip with a thin-film piezoelectric resonator and a mounting substrate having the chip mounted thereon, which makes it possible to prevent a shift or degradation in the electrical properties of the filter attributable to mounting of a chip onto the mounting substrate and to achieve improved reliability of electrical connection between the chip and the mounting substrate, and also to provide a duplexer that includes this piezoelectric resonant filter and methods of manufacturing such filters and duplexers.
A piezoelectric resonant filter of the invention is a filter including a thin-film piezoelectric resonator, the thin-film piezoelectric resonator having a piezoelectric thin film with a piezoelectric property and two exciting electrodes disposed on both surfaces of the piezoelectric thin film to apply an exciting voltage to the piezoelectric thin film. The piezoelectric resonant filter comprises a chip having the thin-film piezoelectric resonator, and a mounting substrate on which the chip is mounted.
The chip has a plurality of chip-side conductors made of a metal, the chip-side conductors being connected to the exciting electrodes or constituting the exciting electrodes. The mounting substrate has a plurality of substrate-side conductors that are made of a metal and to be electrically connected to the chip-side conductors. The piezoelectric resonant filter further comprises a plurality of bumps provided on the chip-side conductors or on the substrate-side conductors. The chip is mounted on the mounting substrate by flip chip bonding such that the chip-side conductors and the substrate-side conductors are electrically and mechanically connected to each other via the bumps. The bumps provided on the chip-side conductors are bonded to the substrate-side conductors, or the bumps provided on the substrate-side conductors are bonded to the chip-side conductors, by interdiffusion between atoms of the respective metals of which the bumps and the conductors are made, without involving melting of the respective metals of which the bumps and the conductors are made.
In the piezoelectric resonant filter of the invention, the chip having the thin-film piezoelectric resonator is mounted on the mounting substrate by flip chip bonding. Therefore, no extra parasitic inductance will develop when the chip is mounted. Furthermore, in the piezoelectric resonant filter of the invention, the bumps provided on the chip-side conductors are bonded to the substrate-side conductors, or the bumps provided on the substrate-side conductors are bonded to the chip-side conductors, by interdiffusion between atoms of the respective metals of which the bumps and the conductors are made, without involving melting of the respective metals of which the bumps and the conductors are made. Accordingly, the thin-film piezoelectric resonator cannot be contaminated by any flux residue or the like.
In the piezoelectric resonant filter of the invention, the bumps may be made of gold.
In the piezoelectric resonant filter of the invention, the chip may have a conductor layer disposed between each of the chip-side conductors and each of the bumps provided on the chip-side conductors, the conductor layer being made of a metal different from the respective metals of which the bumps and the conductors are made. In this case, the conductor layer may be made of titanium or nickel.
In the piezoelectric resonant filter of the invention, the chip may include a series resonator and a parallel resonator each formed of the thin film piezoelectric resonator, the series resonator and the parallel resonator constituting a ladder filter circuit.
A method of manufacturing the piezoelectric resonant filter of the invention comprises the steps of: fabricating the chip; fabricating the mounting substrate; forming a plurality of bumps on the chip-side conductors or on the substrate-side conductors; and mounting the chip on the mounting substrate by flip chip bonding such that the chip-side conductors and the substrate-side conductors are electrically and mechanically connected to each other via the bumps. In the step of mounting, the bumps provided on the chip-side conductors are bonded to the substrate-side conductors, or the bumps provided on the substrate-side conductors are bonded to the chip-side conductors, by interdiffusion between atoms of the respective metals of which the bumps and the conductors are made, without involving melting of the respective metals of which the bumps and the conductors are made.
In the method of manufacturing the piezoelectric resonant of the invention, in the step of mounting, an ultrasonic wave may be applied to the bumps to accelerate the interdiffusion between the atoms of the respective metals.
A duplexer of the invention is connected to an antenna, and comprises a first filter that allows a transmission signal to pass therethrough and interrupts a reception signal, and a second filter that allows the reception signal to pass therethrough and interrupts the transmission signal. At least one of the first and second filters of the duplexer is the piezoelectric resonant filter of the invention.
A method of manufacturing the duplexer of the invention comprises the step of manufacturing the first filter and the step of manufacturing the second filter. At least one of the steps of manufacturing the first filter and manufacturing the second filter includes the step of manufacturing the piezoelectric resonant filter. The step of manufacturing the piezoelectric resonant filter includes the steps of: fabricating the chip of the piezoelectric resonant filter; fabricating the mounting substrate; forming a plurality of bumps on the chip-side conductors or on the substrate-side conductors; and mounting the chip on the mounting substrate by flip chip bonding such that the chip-side conductors and the substrate-side conductors are electrically and mechanically connected to each other via the bumps. In the step of mounting, the bumps provided on the chip-side-conductors are bonded to the substrate-side conductors, or the bumps provided on the substrate-side conductors are bonded to the chip-side conductors, by interdiffusion between atoms of the respective metals of which the bumps and the conductors are made, without involving melting of the respective metals of which the bumps and the conductors are made.
In the method of manufacturing the duplexer of the invention, in the step of mounting, an ultrasonic wave may be applied to the bumps to accelerate the interdiffusion between the atoms of the metals.
Other objects, features and advantages of the invention will become sufficiently clear from the following description.